7 research outputs found
Impact of urban canopy meteorological forcing on aerosol concentrations
The regional climate model RegCM4 extended with the land surface model CLM4.5
was coupled to the chemistry transport model CAMx to analyze the impact of
urban meteorological forcing on surface fine aerosol (PM2.5)
concentrations for summer conditions over the 2001–2005 period, focusing on
the area of Europe. Starting with the analysis of the meteorological
modifications caused by urban canopy forcing, we found a significant increase
in urban surface temperatures (up to 2–3 K), a decrease of specific humidity (by
up to 0.4–0.6 gkg−1), a reduction of wind speed (up to −1 ms−1) and an enhancement of vertical turbulent diffusion coefficient
(up to 60–70 m2s−1).These modifications translated into significant changes in surface aerosol
concentrations that were calculated by a cascading experimental approach.
First, none of the urban meteorological effects were considered. Then, the
temperature effect was added, then the humidity and the wind, and finally, the
enhanced turbulence was considered in the chemical runs. This facilitated the
understanding of the underlying processes acting to modify urban aerosol
concentrations. Moreover, we looked at the impact of the individual aerosol
components as well. The urbanization-induced temperature changes resulted in
a decrease of PM2.5 by −1.5 to −2 µg m−3, while decreased
urban winds resulted in increases by 1–2 µg m−3. The enhanced
turbulence over urban areas resulted in decreases of PM2.5 by −2 µg m−3. The combined effect of all individual impact depends on
the competition between the partial impacts and can reach up to −3 µg m−3 for some cities, especially when the temperature impact was stronger
in magnitude than the wind impact. The effect of changed humidity was found
to be minor. The main contributor to the temperature impact is the
modification of secondary inorganic aerosols, mainly nitrates, while the wind
and turbulence impact is most pronounced in the case of primary aerosol (primary
black and organic carbon and other fine particle matter). The overall as well
as individual impacts on secondary organic aerosol are very small, with the
increased turbulence acting as the main driver. The analysis of the vertical
extent of the aerosol changes showed that the perturbations caused by urban
canopy forcing, besides being large near the surface, have a secondary
maximum for turbulence and wind impact over higher model levels, which is
attributed to the vertical extent of the changes in turbulence over urban
areas. The validation of model data with measurements showed good agreement,
and we could detect a clear model improvement in some areas when including
the urban canopy meteorological effects in our chemistry simulations.</p
Interannual variability in the gravity wave drag – vertical coupling and possible climate links
Gravity wave drag (GWD) is an important driver of the middle
atmospheric dynamics. However, there are almost no observational
constraints on its strength and distribution (especially
horizontal). In this study we analyze orographic GWD (OGWD) output
from Canadian Middle Atmosphere Model simulation with specified
dynamics (CMAM-sd) to illustrate the interannual variability in the
OGWD distribution at particular pressure levels in the stratosphere
and its relation to major climate oscillations. We have found
significant changes in the OGWD distribution and strength depending
on the phase of the North Atlantic Oscillation (NAO), quasi-biennial
oscillation (QBO) and El Niño–Southern Oscillation. The
OGWD variability is shown to be induced by lower-tropospheric wind
variations to a large extent, and there is also significant
variability detected in near-surface momentum fluxes. We argue
that the orographic gravity waves (OGWs) and gravity waves (GWs) in general can be
a quick mediator of the tropospheric variability into the
stratosphere as the modifications of the OGWD distribution can
result in different impacts on the stratospheric dynamics during
different phases of the studied climate oscillations
Modeling the regional impact of ship emissions on NOx and ozone levels over the Eastern Atlantic and Western Europe using ship plume parameterization
In general, regional and global chemistry transport
models apply instantaneous mixing of emissions into
the modelâÂÂs finest resolved scale. In case of a concentrated
source, this could result in erroneous calculation of the evolution of both primary and secondary chemical species. Several studies discussed this issue in connection with emissions from ships and aircraft. In this study, we present an approach to deal with the non-linear effects during dispersion of NOx emissions from ships. It represents an adaptation of the original approach developed for aircraft NOx emissions, which uses an exhaust tracer to trace the amount of the emitted species in the plume and applies an effective reaction rate for the ozone production/destruction during the plumeâÂÂs dilution into the background air. In accordance with previous studies examining the impact of international shipping on the composition of the troposphere, we found that the contribution of ship induced surface NOx to the total reaches 90% over remote ocean and makes 10âÂÂ30% near coastal regions.
Due to ship emissions, surface ozone increases by up to 4âÂÂ
6 ppbv making 10% contribution to the surface ozone budget.
When applying the ship plume parameterization, we show
that the large scale NOx decreases and the ship NOx contribution is reduced by up to 20âÂÂ25%. A similar decrease
was found in the case of O3. The plume parameterization
suppressed the ship induced ozone production by 15âÂÂ30%
over large areas of the studied region. To evaluate the presented parameterization, nitrogen monoxide measurements over the English Channel were compared with modeled values and it was found that after activating the parameterization the model accuracy increases
Revisiting internal gravity waves analysis using GPS RO density profiles: comparison with temperature profiles and application for wave field stability study
We revise selected findings regarding the utilization of Global
Positioning System radio occultation (GPS RO) density profiles for
the analysis of internal gravity waves (IGW), introduced by
Sacha et al. (2014). Using various GPS RO datasets, we show that the
differences in the IGW spectra between the dry-temperature and
dry-density profiles that were described in the previous study as
a general issue are in fact present in one specific data version
only. The differences between perturbations in the temperature and
density GPS RO profiles do not have any physical origin, and there is
not the information loss of IGW activity that was suggested in
Sacha et al. (2014). We investigate the previously discussed question
of the temperature perturbations character when utilizing GPS RO
dry-temperature profiles, derived by integration of the hydrostatic
balance. Using radiosonde profiles as a proxy for GPS RO, we provide
strong evidence that the differences in IGW perturbations between
the real and retrieved temperature profiles (which are based on the
assumption of hydrostatic balance) include a significant
nonhydrostatic component that is present sporadically and might be
either positive or negative. The detected differences in related
spectra of IGW temperature perturbations are found to be mostly
about ±10 %.
The paper also presents a detailed study on the utilization of GPS
RO density profiles for the characterization of the wave field
stability. We have analyzed selected stability parameters derived
from the density profiles together with a study of the vertical
rotation of the wind direction. Regarding the Northern Hemisphere
the results point to the western border of the Aleutian high, where
potential IGW breaking is detected. These findings are also
supported by an analysis of temperature and wind velocity
profiles. Our results confirm advantages of the utilization of the
density profiles for IGW analysis
On the intermittency of orographic gravity wave hotspots and its importance for middle atmosphere dynamics
When orographic gravity waves (OGWs) break, they dissipate their momentum and energy and thereby influence the thermal and dynamical structure of the atmosphere. This OGW forcing mainly takes place in the middle atmosphere. It is zonally asymmetric and strongly intermittent. So-called “OGW hotspot regions” have been shown to exert a large impact on the total wave forcing, in particular in the lower stratosphere (LS). Motivated by this we investigate the asymmetrical distribution of the three-dimensional OGW drag (OGWD) for selected hotspot regions in the specified dynamics simulation of the chemistry-climate model CMAM (Canadian Middle Atmosphere Model) for the period 1979–2010. As an evaluation, we first compare zonal mean OGW fluxes and GW drag (GWD) of the model simulation with observations and reanalyses in the Northern Hemisphere. We find an overestimation of GW momentum fluxes and GWD in the model's LS, presumably attributable to the GW parameterizations which are tuned to correctly represent the dynamics of the Southern Hemisphere. In the following, we define three hotspot regions which are of particular interest for OGW studies, namely the Himalayas, the Rocky Mountains and East Asia. The GW drags in these hotspot regions emerge as strongly intermittent, a result that can also quantitatively be corroborated with observational studies. Moreover, a peak-detection algorithm is applied to capture the intermittent and zonally asymmetric character of OGWs breaking in the LS and to assess composites for the three hotspot regions. This shows that LS peak OGW events can have opposing effects on the upper stratosphere and mesosphere depending on the hotspot region. Our analysis constitutes a new method for studying the intermittency of OGWs, thereby facilitating a new possibility to assess the effect of particular OGW hotspot regions on middle atmospheric dynamics.</p
On the aliasing of the solar cycle in the lower stratospheric tropical temperature
©2017. American Geophysical Union. All Rights Reserved. The double-peaked response of the tropical stratospheric temperature profile to the 11 year solar cycle (SC) has been well documented. However, there are concerns about the origin of the lower peak due to potential aliasing with volcanic eruptions or the El Niño–Southern Oscillation (ENSO) detected using multiple linear regression analysis. We confirm the aliasing using the results of the chemistry-climate model (CCM) SOCOLv3 obtained in the framework of the International Global Atmospheric Chemisty/Stratosphere-troposphere Processes And their Role in Climate Chemistry-Climate Model Initiative phase 1. We further show that even without major volcanic eruptions included in transient simulations, the lower stratospheric response exhibits a residual peak when historical sea surface temperatures (SSTs)/sea ice coverage (SIC) are used. Only the use of climatological SSTs/SICs in addition to background stratospheric aerosols removes volcanic and ENSO signals and results in an almost complete disappearance of the modeled solar signal in the lower stratospheric temperature. We demonstrate that the choice of temporal subperiod considered for the regression analysis has a large impact on the estimated profile signal in the lower stratosphere: at least 45 consecutive years are needed to avoid the large aliasing effect of SC maxima with volcanic eruptions in 1982 and 1991 in historical simulations, reanalyses, and observations. The application of volcanic forcing compiled for phase 6 of the Coupled Model Intercomparison Project (CMIP6) in the CCM SOCOLv3 reduces the warming overestimation in the tropical lower stratosphere and the volcanic aliasing of the temperature response to the SC, although it does not eliminate it completely